3-dimensional (“3D”) inspection and/or metrology of silicon wafers are required processes in the semiconductor industry. Such inspection can be used, for example, to test the through silicon via (“TSV”) and bump structure or the particle shape (e.g., size and height). Typical techniques for inspection or metrology include: (1) triangulation; (2) geometric shadow; (3) various confocal microscope techniques; and (4) white-light (or broadband light) interferometry. Triangulation and geometric shadow techniques are not precise enough for contemporary back-end of line (“BEOL”) applications. Confocal microscopy and interferometry techniques typically fail to meet throughput requirements.
White-light interferometry is known to be a high-resolution method for 3D inspection and metrology and has been used in the semiconductor industry. There are two types of such devices offering in the market: (1) scanning white-light interferometers (“SWI”); and (2) spectroscopic white-light interferometers. In SWI devices, either the sample (e.g., the wafer under inspection) or the inspection optics scan along a direction perpendicular to the wafer surface, the z-direction, for a distance. Multiple frames are taken at specific z-values to determine the height measurement for a specific x-y location on the wafer surface. Such SWI devices are robust but generally slow. Furthermore, this technique requires the sample to move to field of view and stabilized before a measurement is taken—another factor limiting its speed.
In spectrographic devices, reflected light is detected with a spectrometer without the need for mechanical scanning in the z-direction. Such devices are constructed as point-scan devices that scan along a line or within an area. Speeds are often limited by light-utilization efficiency. Another disadvantage of point-scan techniques is a loss in accuracy due to the sensitivity of such point measurements to environmental conditions, such as vibration of the scanning mechanism. This is because such systems cannot remove changes in height from one point to another point caused by environmental externalities.